5Asexual ReproductionAsexual reproduction is reproduction that involves a single parent producing an offspring.The offspring produced are, in most cases, genetically identical to the single cell that produced them.Asexual reproduction is a simple, efficient, and effective way for an organism to produce a large number of offspring.Prokaryotic organisms (like bacteria) reproduce asexually, as do some eukaryotes (like sponges)

6Sexual ReproductionIn sexual reproduction, offspring are produced by the fusion of two sex cells – one from each of two parents. These fuse into a single cell before the offspring can grow.The offspring produced inherit some genetic information from both parents.Most animals and plants, and many single-celled organisms, reproduce sexually.

8Cell DivisionCells duplicate their genetic material before they divide, ensuring that each daughter cell receives an exact copy of the genetic material, DNA.A dividing cell duplicates its DNA, allocates the two copies to opposite ends of the cell, and only then splits into daughter cells.

9Cellular Organization of the Genetic MaterialA cell’s endowment of DNA (its genetic information) is called its genome.DNA molecules in a cell are packaged into chromosomes.

10ChromosomesThe genetic information that is passed on from one generation of cells to the next is carried by chromosomes.Every cell must copy its genetic information before cell division begins.Each daughter cell gets its own copy of that genetic information.Cells of every organism have a specific number of chromosomes.

11Prokaryotic ChromosomesProkaryotic cells lack nuclei. Instead, their DNA molecules are found in the cytoplasm.Most prokaryotes contain a single, circular DNA molecule, or chromosome, that contains most of the cell’s genetic information.

12Eukaryotic ChromosomesIn eukaryotic cells, chromosomes are located in the nucleus, and are made up of chromatin.

15The nucleosomes interact with one another to form coils and supercoils that make up chromosomes.

16Chromosomes During Cell DivisionIn preparation for cell division, DNA is replicated and the chromosomes condenseEach duplicated chromosome has two sister chromatids, which separate during cell divisionThe centromere is the narrow “waist” of the duplicated chromosome, where the two chromatids are most closely attached

21S Phase: DNA ReplicationIn the S (or synthesis) phase, new DNA is synthesized when the chromosomes are replicated.

22G2 Phase: Preparing for Cell DivisionIn the G2 phase, many of the organelles and molecules required for cell division are produced.

23M Phase: Cell DivisionIn eukaryotes, cell division occurs in two stages: mitosis and cytokinesis.Mitosis is the division of the cell nucleus.Cytokinesis is the division of the cytoplasm.

24Important Cell Structures Involved in MitosisChromatid – each strand of a duplicated chromosomeCentromere – the area where each pair of chromatids is joinedCentrioles – tiny structures located in the cytoplasm of animal cells that help organize the spindleSpindle – long proteins (part of the cytoskeleton) that the centrioles produceHelps move the chromosomes into place.

40Cytokinesis in Animal CellsIn plants, the cell membrane is not flexible enough to draw inward because of the rigid cell wall.Instead, a cell plate forms between the divided nuclei that develops into cell membranes.A cell wall then forms in between the two new membranes.

42LE 12-10 Chromatin condensing Nucleus Chromosomes Cell plate 10 µmNucleolusProphase. Thechromatin is condensing.The nucleolus is beginning to disappear.Although not yet visible in the micrograph, the mitotic spindle is starting to form.Prometaphase. Wenow see discrete chromosomes; eachconsists of two identical sister chromatids. Laterin prometaphase, thenuclear envelope will fragment.Metaphase. The spindle is complete, and the chromosomes, attached to microtubules at their kinetochores, are all atthe metaphase plate.Anaphase. Thechromatids of each chromosome have separated, and the daughter chromosomes are moving to the ends of the cell as their kinetochore micro-tubules shorten.Telophase. Daughter nuclei are forming. Meanwhile, cytokinesis has started: The cell plate, which will divide the cytoplasm in two, is growing toward the perimeter of the parent cell.

50Binary FissionProkaryotes (bacteria and archaea) reproduce by a type of cell division called binary fissionIn binary fission, the chromosome replicates (beginning at the origin of replication), and the two daughter chromosomes actively move apart

52Chromosome replication begins. Soon thereafter,LE 12-11_2Cell wallOrigin ofreplicationPlasmamembraneE. coli cellBacterialchromosomeChromosome replication begins. Soon thereafter,one copy of the origin moves rapidly toward the other end of the cell.Two copiesof originOriginOriginReplication continues. One copy of the origin is now at each end of the cell.

53LE 12-11_3 Cell wall Origin of replication Plasma membraneE. coli cellBacterialchromosomeChromosome replication begins.Soon thereafter,one copy of the origin moves rapidly toward the other end of the cell.Two copiesof originOriginOriginReplication continues. One copy of the origin is now at each end of the cell.Replication finishes.The plasma membrane grows inward, andnew cell wall is deposited.Two daughtercells result.

56The Cell Cycle Control SystemThe sequential events of the cell cycle are directed by a distinct cell cycle control system, which is similar to a clockThe clock has specific checkpoints where the cell cycle stops until a go-ahead signal is received

58For many cells, the G1 checkpoint seems to be the most important oneIf a cell receives a go-ahead signal at the G1 checkpoint, it will usually complete the S, G2, and M phases and divideIf the cell does not receive the go-ahead signal, it will exit the cycle, switching into a nondividing state called the G0 phase

59LE 12-15G0G1 checkpointG1G1If a cell receives a go-ahead signal at the G1 checkpoint, the cell continues on in the cell cycle.If a cell does not receive a go-ahead signal at the G1 checkpoint, the cell exits the cell cycle and goes into G0, a nondividing state.

60An example of external signals is density-dependent inhibition, in which crowded cells stop dividingMost animal cells also exhibit anchorage dependence, in which they must be attached to a substratum (connective tissue) in order to divide

63Loss of Cell Cycle Controls in Cancer CellsCancer cells do not respond normally to the body’s control mechanismsCancer cells form tumors, masses of abnormal cells within otherwise normal tissueIf abnormal cells remain at the original site, the lump is called a benign tumorMalignant tumors invade surrounding tissues and can metastasize, exporting cancer cells to other parts of the body, where they may form secondary tumors